Indirectly heated cathode (IHC) ion sources operate by supplying a current to a filament disposed behind a cathode. The filament emits thermionic electrons, which are accelerated toward and heat the cathode, in turn causing the cathode to emit electrons into the ion source chamber. The cathode is disposed at one end of the ion source chamber. A repeller is typically disposed on the end of the ion source chamber opposite the cathode. The repeller may be biased so as to repel the electrons, directing them back toward the center of the ion source chamber. In some embodiments, a magnetic field is used to further confine the electrons within the ion source chamber. The electrons cause a plasma to be created. Ions are then extracted from the ion source chamber through an extraction aperture.
The ion source chamber is typically made of an electrically conductive material, which has good electrical conductivity and a high melting point. The ion source chamber may be maintained at a certain electrical potential. Additionally, the cathode and the repeller are disposed within the ion source chamber, and are typically maintained at electrical potentials that are different from the ion source chamber. Further, apertures are created in the walls of the ion source chamber to allow electrical connections to the cathode and the repeller. These apertures are sized such that arcing does not occur between the wall of the ion source chamber and the electrical connections to the cathode and repeller. These apertures, however, also allow feed gas, which is introduced into the ion source chamber, to escape.
Additionally, the materials used to make the ion source chamber may also have good thermal conductivity as one function of the ion source chamber may be to remove heat from within the chamber via conduction to a cooler surface.
Thus, the materials used for the ion source chamber typically have high melting points, good electrical conductivity and good thermal conductivity. In some embodiments, materials such as tungsten and molybdenum are used to construct the ion source chamber.
One issue associated with IHC ion sources is that the material used to construct the ion source chamber may be expensive and difficult to machine. Additionally, the ions generated within the ion source chamber may cause particles of the ion source chamber to be removed and introduced into the extracted ion beam. Thus, the material used to create the ion source chamber may introduce contamination into the extracted ion beam. Further, feed gas is lost through the apertures that are created to allow electrical connections to the cathode and repeller.
Therefore, an IHC ion source in which the material used to construct the ion source chamber did not contaminate the ion beam would be advantageous. Further, it would be beneficial if the openings used to provide electrical connection to the cathode and repeller could be reduced in size or eliminated, so as to reduce the flow of feed gas escaping from the ion source chamber.